JP2003064418A - METHOD FOR PRODUCING X70 CLASS STEEL SHEET WITH SHEET THICKNESS OF <=15 mm HAVING HIGH IMPACT ABSORBED ENERGY AS NONWATER COLD-ROLLED - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a steel sheet with a sheet thickness of <=15 mm which has strength in an X70 class and vE-20 of >=250J as it is cold-rolled without water. SOLUTION: In the method for producing an X70 class steel sheet with a sheet thickness of <=15 mm which has high impact absorbed energy, a slab having chemical components containing, by mass, 0.03 to 0.06% C, 1.4 to 2.0% Mn, 0.05 to 0.5% Mo, 0.01 to 0.1% Nb, <=0.01% P, <=0.003% S and <=0.005% O, and further containing one or more metals selected from 0.05 to 0.5% Si, 0.001 to 0.05% Al and 0.005 to 0.05% Ti, and the balance iron with inevitable impurities is subjected to hot rolling so as to control the thickness of the slab before the rolling to >=10 times that of the steel sheet after the rolling, and to be heated to >=1,000 deg.C. In this case, the rolling is finished so that the cumulative draft is controlled to 20 to <60% in the temperature range of Ar3 to Ar3 +100 deg.C, and, after that, air cooling is applied therefor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a technique for producing an X70 grade steel sheet having a sheet thickness of 15 mm or less and having a high absorbed energy at the time of ductile fracture with high productivity. In the steel industry, it is applied to the plate manufacturing process. The steel sheet produced according to the present invention is mainly used for a line pipe for transportation of crude oil, natural gas and the like. In addition, it can be applied to various steel structures in which ductile fracture characteristics are important.

[0002]

2. Description of the Related Art In the field welding of line pipes, there is a demand for thinning from the viewpoint of welding efficiency so that the number of welding passes is small. On the other hand, from the viewpoint of transportation efficiency, there is a demand for higher strength in order to increase the operating pressure of the line pipe. In addition, in recent years, from the viewpoint of safety of line pipes, there is a demand for increasing resistance to unstable ductile fracture. As steel plates for line pipes, which are required to have high dimensions, it is required to provide steel plates that satisfy the following specifications, for example. Plate thickness ≤15 mm X70 grade strength (API standard) Absorbed energy in 2 mm V-notch full size Charpy impact test at -20 ° C (vE _-20 ) ≥250J

The present invention is a technique for manufacturing a steel plate satisfying the above-mentioned three specifications with high productivity.

In the thick plate manufacturing process, it is widely practiced to apply accelerated cooling after rolling in order to enhance the strength and toughness of the steel plate. If you make full use of such accelerated cooling technology,
It is not impossible to manufacture steel sheets that combine the above-mentioned required characteristics. However, when accelerated cooling is applied to a very thin steel plate having a plate thickness of 15 mm or less, there is a problem that the cooling becomes uneven and the steel plate shape deteriorates. As a result, an additional work process for correcting the shape of the steel sheet occurs, and the productivity at the production site is significantly impaired. Against this background, a new mass production technology is required to manufacture a steel sheet having the above-mentioned required characteristics to a high production level without using the accelerated cooling as it is in the non-water-cooled rolling. The manufacturing method without cooling is referred to as non-water cooling rolling or non-water cooling type rolling).

[0005]

According to the present invention, the plate thickness is 15
mm or less, X70 grade strength, 250 J or more v
The present invention provides a method for producing a steel sheet having E- ₂₀ as it is in non-water-cooled rolling.

[0006]

Means for Solving the Problems The present inventor can control the ductile fracture rate at −20 ° C. to 100% by controlling the metallographic structure of the steel sheet from both aspects of steel composition and rolling conditions, and
The inventors have found that the fracture resistance can be increased under a ductile fracture surface ratio of 100%, and as a result, an X70 grade steel sheet having a thickness of 15 mm or less and having high impact absorption energy can be produced, and the present invention has been completed.

The gist of the present invention is as follows.

(1) C in mass%: 0.03 to 0.0
6%, Mn: 1.4-2.0%, Mo: 0.05-0.
5%, Nb: 0.01 to 0.1%, P: ≤ 0.01
%, S: ≤ 0.003%, O: ≤ 0.005%, Si: 0.05-0.5%, Al: 0.0
01-0.05%, Ti: 0.005-0.05% 1
Steel plate after the rolling, when a steel slab containing two or more kinds, and the balance being iron and unavoidable impurities, is hot-rolled by heating it to 1000 ° C. or higher. 10 times or more of the thickness, Ar _{3 to} Ar ₃ +100
A plate thickness of 15 mm having high impact absorption energy, which is characterized in that rolling is finished so that the cumulative reduction amount becomes 20% or more and less than 60% in a temperature range of ℃, and then air cooling is performed.
The following non-water-cooled manufacturing method of X70 grade steel sheet.

(2) Cu: ≤1%, Ni: ≤% by mass%
1%, Cr: ≤ 1%, V: ≤ 0.1%, B: ≤ 0.00
5% of 1 type (s) or 2 or more types are contained, The non-water-cooled manufacturing method of the X70 grade steel plate with a plate thickness of 15 mm or less which has high impact absorption energy as described in said (1).

(3) Ca in mass%: ≤0.005%,
Mg: ≤ 0.005%, REM: ≤ 0.01%, Zr:
Non-water cooling type X70 grade steel sheet having a thickness of 15 mm or less and having a high impact absorption energy according to the above (1) or (2), characterized by containing ≤0.01% of one type or two or more types. Production method.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The greatest technical problem in the present invention is to stably obtain vE- ₂₀ of 250 J or more under mass production. The technical idea for this purpose will be roughly classified into (1) and (2).

(1) First, it is the minimum condition for obtaining high vE- ₂₀ that the fracture mode at the Charpy test temperature of -20 ° C is made to be a complete ductile fracture. That is, -2
The ductile fracture rate at 0 ° C must be 100%.
For this purpose, in the steel material targeted by the present invention, the fracture surface transition temperature needs to be -60 ° C or lower. In order to realize this, in the present invention, the metal structure of the steel sheet is refined under the following three conditions. A steel slab to which 0.01% or more of Nb is added is heated to 1000 ° C. or higher and then hot rolled. The cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C. in hot rolling is set to 20% or more. The thickness of the billet before hot rolling is 10 times the thickness of the steel sheet after hot rolling.
More than double.

By satisfying these three conditions at the same time, the austenite (γ) structure during rolling is refined, the transformation structure generated in the subsequent cooling process is refined, and the fracture surface transition temperature of -60 ° C or less is achieved. The ductile fracture surface ratio at −20 ° C. is stably 100%. If these three conditions are not met, the fracture surface transition temperature of the steel sheet becomes higher than −60 ° C., and it becomes difficult to stably secure a ductile fracture surface ratio of 100% at −20 ° C. About 0.01% Nb
When the steel billet heating temperature is less than 1000 ° C. or the billet heating temperature is less than 1000 ° C., the amount of Nb dissolved in γ is insufficient. Do not miniaturize. With respect to the above, if the cumulative reduction amount in Ar _{3 to} Ar ₃ + 100 ° C. is less than 20%, the workability of γ in the γ non-recrystallization temperature region is insufficient, and γ is not sufficiently refined. With respect to the above, if the thickness of the billet before rolling is less than 10 times the thickness of the steel sheet after rolling, the working amount at a high temperature of Ar ₃ + 100 ° C. or higher will be insufficient and γ will not be sufficiently refined.

(2) Next, the fracture resistance must be increased under a ductile fracture surface ratio of 100%. For this reason, the present invention has been devised to control the metallographic structure from both aspects of steel composition and rolling conditions, focusing on the suppression of separation.
Separation is a crack that is perpendicular to the fracture surface and parallel to the rolling surface. It is widely known that absorption energy decreases when separation occurs. 1 to which the present invention is directed
When a thin steel sheet of 5 mm or less is produced by non-water-cooled rolling, if strong control rolling is performed with emphasis on the fracture surface transition temperature as in the conventional case, a large number of separations occur and the ductile fracture surface ratio is 100%. However, it was difficult to stably obtain high absorbed energy exceeding 250 J. As a cause of separation, the involvement of rolling texture is widely known, as described in, for example, “Iron and Steel, 68 (1982), 435”. And
It is known that ending the rolling with Ar ₃ or more in order to suppress the development of the rolling texture is effective for suppressing the separation. However, there was no knowledge about the effect of steel components on the occurrence of separation.

Therefore, as a result of detailed examination of the effect of the steel composition on the separation, the inventors have found out the experimental fact that the C content has a very large effect. FIG. 1 shows a transition curve of the Charpy impact characteristics of a 12 mm-thick X70 grade steel sheet in which the amount of C is changed. These steel sheets were rolled and air-cooled at the same temperature higher than Ar _{3 of} all steels.

All of these steels have a fracture surface transition temperature of -60 ° C or less, and the ductile fracture surface ratio at -20 ° C is 100% for all steel sheets. However, the value of vE _-20 varies greatly depending on the amount of C. When the C content is 0.055% or less, it exceeds 300 J, and when the C content is 0.074% or more, it is 25
Below 0J. The reason why vE _-20 fluctuates depending on the amount of C is that the amount of separation generated depends on the amount of C.

Amount of separation generated at −20 ° C. (I
s: the total length of the separation length that appeared on the fracture surface), no separation occurred at a low C content of 0.055% or less. On the other hand, high C of 0.074% or more
Separation occurs when the volume is reached. At −40 ° C., it is clear that the amount of separation generation gradually increases as the amount of C increases. That is, the ductility fracture rate is 100.
It has been found that reducing the amount of C is extremely effective for suppressing the separation under%, and obtaining a high absorbed energy.

When the amount of C is small, the development of the rolling texture is suppressed and the separation hardly occurs. The second effect of reducing the C content is that the separation that occurs along the center segregated portion of the steel sheet is suppressed. Since the separation of this form is deeply formed perpendicularly to the fracture surface, the absorbed energy is greatly impaired. When the C content is small, the center segregation in the continuously cast steel piece is reduced, so that the development of the band structure generated in the center segregation portion of the steel sheet is suppressed, and the deep separation due to the center segregation is less likely to occur.

As described above, it was found that by reducing the amount of C, the occurrence of separation can be strongly suppressed over the entire region from the surface layer in the plate thickness direction to the inside. Based on this completely new finding, in the present invention, the C content is 0.06%.
Controlling the following is a technical pillar.

Further, as a result of studying the relationship between the rolling conditions and the separation, even if it is attempted to avoid the rolling texture by ending the rolling with Ar ₃ or more as conventionally known,
It has been found that when the cumulative reduction amount of Ar _{3 to} Ar ₃ + 100 ° C. is 60% or more, the rolling texture of γ is inherited to the transformation structure and separation easily occurs. Therefore, in the present invention, the cumulative reduction amount in this temperature range is suppressed to less than 60% to thoroughly suppress the occurrence of separation.

Next, the reasons for limiting each chemical component will be described.

C is the most important element in the present invention. To secure the strength, 0.03% or more of C is necessary.
However, when the amount of C is large, separation is likely to occur and the absorbed energy is lowered. Further, when the amount of C is large, center segregation is promoted, and deep separation due to this is generated to reduce the absorbed energy. Furthermore, C
As the content of cementite particles and pearlite phase increases, the volume ratio of cementite particles and pearlite phase becomes buds of void generation in ductile fracture, which promotes destruction and reduces absorbed energy. From the above, the upper limit of C must be 0.06%.

Mn is an element indispensable for securing strength and toughness, and particularly from the viewpoint of strength, it has to be positively added in place of the low C of the present invention. It is necessary to add 1.4% or more of Mn in order to secure X70 grade strength under low C. When Mn exceeds 2.0%, center segregation is promoted, deep separation resulting from this is generated, and absorbed energy is reduced. Therefore, the upper limit of Mn is 2.0
%.

Mo is a very important element in the present invention.
Mo enhances hardenability in the transformation after rolling and promotes the formation of a fine structure in which acicular ferrite and bainite are mixed.
At the same time, it prevents the formation of a band structure parallel to the rolling direction and promotes the generation of a more isotropic structure. The dispersed state of the cementite particles is also miniaturized. As a result, the microstructure refinement lowers the fracture surface transition temperature and increases the strength.
Furthermore, separation and voids are less likely to occur, and absorbed energy is improved. In order to enjoy these effects, 0.05% or more of Mo is necessary. Mo is 0.
If it exceeds 5%, the hardenability becomes excessive and MA (Marte
nite austenite constitue
nt) increases the hardening phase and reduces the absorbed energy. Therefore, the upper limit of Mo is set to 0.5%.

Nb is an important element in the present invention. Nb promotes the refinement of the γ structure by rolling and refines the transformation structure. As a result, the fracture surface transition temperature is lowered and the strength is increased. Strength is also increased by precipitation hardening. For these, 0.01% or more of Nb is necessary. When Nb exceeds 0.1%, central segregation is promoted, and deep separation occurs due to this, and absorbed energy decreases. Therefore, the upper limit of Nb is set to 0.1%.

P is an element which is not preferred in the present invention.
P promotes central segregation and segregates at the grain boundaries to cause a significant deterioration in toughness. To obtain high absorbed energy, P must be 0.01% or less.

S and O are elements which are not preferred in the present invention. These form non-metallic inclusions, promote the generation of voids, and lower the absorbed energy. S must be 0.003% or less and O must be 0.005% or less.

Si, Al and Ti act as deoxidizing elements. In order to make O 0.005% or less, these 1
It is necessary to add more than one seed. For this reason, Si is 0.
05% or more, Al 0.001% or more, Ti 0.00
5% or more is required. If the amount of these deoxidizing elements is too large, the oxide becomes coarse and adversely affects the starting point of fracture. Therefore, Si is 0.5%, Al is 0.05%, and Ti is 0.0%.
The upper limit is 5%.

Cu, Ni, Cr, V and B are effective in increasing the strength. If the addition amount of these is too large, the HAZ toughness is impaired, so Cu is 1%, Ni is 1%, and Cr is 1%.
%, V is 0.1%, and B is 0.005%.

Ca, Mg, REM and Zr form sulfides in preference to Mn and form spherical inclusions that are difficult to stretch by rolling. As a result, separation and voids are less likely to occur, and absorbed energy is improved. If the amount of these desulfurizing elements is too large, the sulfide becomes coarse and adversely affects the starting point of fracture, so 0.005% for Ca and 0.0 for Mg.
The upper limit is 05%, REM is 0.01%, and Zr is 0.01%.

The conditions for hot rolling a steel slab characterized by the above low C-high Mn-Mo-Nb components will be described. First, the heating temperature of the billet is set to 1000 ° C. or higher. The reason for this is that if the heating temperature is less than 1000 ° C., the amount of Nb dissolved in γ will be insufficient, so that the γ structure will not be finely refined by rolling and the precipitation hardening of Nb will be small. . That is, both the strength and the fracture surface transition temperature deteriorate. Next, the cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C. is set to 20% or more and less than 60%. The reason for this is
This is because if the cumulative reduction amount in this temperature range is less than 20%, the γ structure is not sufficiently refined by rolling and the fracture surface transition temperature deteriorates. Further, when the cumulative reduction amount in this temperature range is 60% or more, the texture of γ due to rolling develops and is inherited in the transformation structure, separation easily occurs and the absorbed energy deteriorates. Next, the rolling is finished with Ar ₃ or more. The reason for this is that when the rolling is finished with less than Ar ₃ , the rolling texture is significantly developed due to the formation of worked ferrite, a large amount of separation is generated, and the absorbed energy is drastically reduced. Next, after the rolling is finished, air cooling is performed. The reason for this is that if accelerated cooling is applied to a very thin steel plate having a thickness of 15 mm or less, the cooling becomes uneven and the shape of the steel plate deteriorates, and an extra work process for correcting this occurs. This is because the productivity is significantly impaired. In the above hot rolling, the thickness of the billet before rolling is 10 times or more the thickness of the steel sheet after rolling. The reason for this is that if the thickness of the slab before rolling is less than 10 times the thickness of the steel sheet after rolling, the working amount at Ar ₃ + 100 ° C. or higher will be insufficient and γ will not be sufficiently refined. Is deteriorated.

[0032]

[Example] Using continuously cast steel pieces having the chemical composition shown in Table 1 as a raw material, steel plates having a plate thickness of 15 mm or less were produced under the non-water-cooled rolling condition under the thick plate production conditions shown in Table 2. Table 3 shows the mechanical properties of the steel sheet.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Steels 1 to 8 are steels according to the present invention, which have chemical compositions and
By optimizing the building conditions to a specific narrow range, X70 grade
Strength and high vE over 250J_-20Be satisfied at the same time
There is. Since it is still non-water-cooled, the productivity at this time is high.
Yes. On the other hand, Steels 9 to 24 are conventional steels and have no chemical composition.
Since the manufacturing conditions deviate from the optimum range,
Or the absorbed energy cannot be achieved. steel
No. 9 has a shortage of C and therefore lacks YS. Steel 10 is C
VE because there are many_-20Is running out. Steel 11 has Mn
YS is insufficient due to the small number. Steel 12 has much Mn
For vE_-20Is running out. Steel 13 has little Mo
For vE_-20Is running out. Steel 14 has a lot of Mo
VE_-20Is running out. Steel 15 has less Nb
For YS, FATT, SA_-20, VE_-20Has deteriorated
It Steel 16 has a large amount of Nb, so vE _-20Is running out
It Steel 17 has a lot of P, so FATT and SA_-20Is deteriorated
Then vE _-20Is also lacking. Steel 18 has a lot of S
vE_-20Is running out. Steel 19 is a deoxidizing element Si
O is increased because there is less vE_-20Lack
is doing. Steel 20 is the steel plate thickness after rolling is the thickness of the billet before rolling
FATT and SA because they are small compared to_-20Has deteriorated
vE_-20Is running out. Steel 21 has a low heating temperature
To YS, FATT, SA_-20, VE_-20Is deteriorated.
Steel 22 is Ar₃~ Ar₃Small cumulative reduction at + 100 ° C
Because of YS, FATT, SA_-20, VE_-20Has deteriorated
There is. Steel 23 is Ar₃~ Ar₃Cumulative reduction at + 100 ℃
VE because there are many _-20Is deteriorated. Steel 24 is finished rolling
End temperature is Ar₃VE to be less than_-20Is running out
It

[0037]

According to the present invention, when the plate thickness is 15 mm or less,
It has become possible to manufacture a steel sheet having a strength of X70 grade and a vE- ₂₀ of 250 J or more with high productivity. As a result, it has become possible for the steel plate manufacturer to keep the manufacturing cost low and shorten the manufacturing delivery time. The steel sheet manufactured according to the present invention is applied to various steel structures in which ductile fracture characteristics are important, including line pipes for transporting crude oil, natural gas, etc., and contributes to improving the safety of steel structures. .

[Brief description of drawings]

FIG. 1 is a diagram showing a transition curve of a Charpy impact characteristic in a 12 mm-thick X70 grade steel sheet in which the amount of C is changed.

Continued front page    (72) Inventor Yoshio Terada             1 Kimitsu, Kimitsu-shi Mr. Nippon Steel Corporation             Tsu Steel Works F-term (reference) 4K032 AA01 AA02 AA04 AA08 AA11                       AA14 AA16 AA19 AA22 AA23                       AA26 AA27 AA29 AA31 AA35                       AA36 AA39 AA40 BA01 CA02                       CA03 CB02 CC03 CD05

Claims (3)

[Claims] 1. C: 0.03 to 0.06% by mass%, Mn: 1.4 to 2.0%, Mo: 0.05 to 0.5%, Nb: 0.01 to 0.1. %, P: ≦ 0.01%, S: ≦ 0.003%, O: ≦ 0.005%, further Si: 0.05 to 0.5%, Al: 0.001 to 0.05 %, Ti: 0.005 to 0.05% of one or two or more kinds, and the balance is steel and a slab of a chemical composition in which inevitable impurities are heated to 1000 ° C. or higher and hot rolled. When performing, the thickness of the billet before rolling is 10 times or more the thickness of the steel sheet after rolling, and Ar _{3 to} Ar ₃ +1
Cumulative reduction is 20% or more and 60% in the temperature range of 00 ℃
A plate thickness 15 having high impact absorption energy, which is characterized in that the rolling is finished so as to be less than 1 and then air-cooled.
A non-water-cooled manufacturing method of X70 grade steel sheet having a size of mm or less. 2. Cu: ≤ 1%, Ni: ≤ 1%, Cr: ≤ 1%, V: ≤ 0.1%, B: ≤ 0.005% by mass% and one or more kinds are contained. The plate thickness 1 having high shock absorption energy according to claim 1, characterized in that
A non-water-cooled manufacturing method of an X70 grade steel sheet having a size of 5 mm or less. 3. Content of one or more of Ca: ≦ 0.005%, Mg: ≦ 0.005%, REM: ≦ 0.01%, Zr: ≦ 0.01% in mass%. The method for producing a non-water-cooled X70 grade steel sheet having a high impact absorption energy according to claim 1 or 2 and having a thickness of 15 mm or less.